1. Field of the Invention
The present invention relates to a motor driver capable of detecting insulation deterioration in the motor in a motor drive system for driving the motor using a motor drive amplifier such as a voltage-type inverter or a PWM inverter.
2. Description of the Related Art
The protracted use of a motor deteriorates the motor insulation in accordance with the environment in which it is operated. The leakage current due to the insulation deterioration activates a leakage interrupter, so that the apparatus using the motor may suddenly stop. In such case, it is not clear whether the motor or the driver is a problem, and a considerable time is consumed to trace the cause of the fault. The period during which a system or production line using the motor is suspended tends to be undesirably lengthened.
It has been common practice to detect the insulation deterioration of the motor by detecting the leakage current as described above. The leakage current detector, however, can detect the leakage current only after it reaches some magnitude. Thus, the leakage current is often detected only after the insulation deterioration of the motor has progressed considerably. As the sudden stoppage of the system or apparatus is due to the leakage current, the jobs on the whole apparatus or whole system of the production line, etc. must be suspended to investigate whether the stopping of the apparatus or the system is caused by the motor itself or the motor driver or the peripheral devices. Another problem is that the leakage current detector cannot contribute to the prediction and the preventive maintenance against the insulation deterioration of the motor.
A method of preventing the inoperability of an air conditioning system by discovering the insulation deterioration of the motor of a compressor at an early time has been proposed (Japanese Unexamined Patent Publication No. 2001-141795). In this method, while the motor is stopped, a DC voltage is converted into an arbitrary voltage and frequency and a high-frequency pulse is applied to drive a transistor of the power converter for driving the motor, a DC voltage (or a preset voltage) obtained by rectifying and filtering the three-phase AC power and the motor current are detected, and based on the DC voltage and the motor current thus detected, the insulation resistance is calculated, and an alarm is issued whenever the insulation resistance value decreases to below the set insulation resistance value.
An invention for preventing the malfunction of the leakage interrupter is in the public domain (Japanese Unexamined Patent Publication No. 10-210649), in which, in order to prevent the malfunction of the leakage current interrupter caused by the leakage current flowing through the wiring between the inverter and the motor and the stray capacitance between the conductive portion and the ground in the motor when an inverter is switched at high frequency in the motor driven by a voltage-type inverter, a DC reactor is inserted on the positive and negative lines connecting the rectifier and the inverter, the positive and negative lines are grounded through a series circuit of a capacitor and a resistor, and the zero-phase current generated by the inverter operation flows through a closed circuit of the stray capacitance, the grounding capacitor, a resistor and the positive and negative lines.